As mankind expands his presence and activity throughout the world, he is often limited by the availability of electrical energy to support his endeavors. Fuel Cells offer one solution to this dilemma by directly deriving electricity from chemical feedstocks such as oxygen and hydrogen. The Fuel Cell approach also offers the potential to reduce pollution problems inherent in direct combustion technology. Applications for Fuel Cells include power for vehicular traction, stationary power for home and industry, and power supplies for marine use. However, pure hydrogen fuel is not always available, and the development of distribution means for hydrogen is uncertain.
In order for the Fuel Cell technology to realize the potential as a generic energy source, flexibility in the choice of fuel is needed. Large-scale technology such as Solid Oxide Fuel Cells (SOFC) and Phosphoric Acid Fuel Cells (PAFC) achieve some feed flexibility by operating at high temperatures, and thus xe2x80x9cburnxe2x80x9d some of the anode contaminants that typically result from deriving hydrogen from carbon-containing feedstocks such as methane or propane. Both PAFC and SOFC technology are not amenable to the smaller scales (approximately  less than 200 Kwatts) envisioned for automotive, and other applications cited above.
The Polymer Electrolyte Membrane Fuel Cell (PEMFC) is often cited as the appropriate energy source for applications requiring less than around 200 kWatts, and also for devices needing as little as a few hundred watts. This class of fuel cell operates at less than 180xc2x0 C., and more typically around 70xc2x0 C. due to the limitations in the stability of the polymer electrolyte membrane. There is great enthusiasm behind the PEMFC approach based on this system""s lack of liquid electrolyte, ease of construction, and high specific power as a function of volume or mass.
In order to impart some fuel flexibility for the PEMFC, an additional fuel-reforming component is needed. The xe2x80x9creformerxe2x80x9d converts hydrogen-containing substances such as methane, propane, methanol, ethanol, and gasoline into hydrogen gas, carbon monoxide, and carbon dioxide through either a steam reformation reaction, partial oxidation, or a combination of both. Reformer technology has now advanced to the state whereby commercially units are available. For example, a newly formed company Epyx (Acorn Park, Cambridge, Mass.) offers a fuel processor that converts gasoline into hydrogen. Johnson Matthey PLC (London, UK) offers a HotSpot(trademark) fuel processor that converts methanol using a combination of steam reforming and partial oxidation. For both these technologies, the untreated output is hydrogen and approximately 1-2% carbon monoxide. Through additional clean-up, the carbon monoxide can be reduced to around 50 ppm or less.
Platinum has long been acknowledged as the best anode catalyst for hydrogen. Early fuel cells employed particles of platinum black mixed with a binder as a component in gas diffusion electrodes. The use of platinum black for hydrogen has been largely supplemented by the highly disperse and very active catalysts created by the methods similar to that found in Petrow and Allen, U.S. Pat. No. 4,082,699. This patent teaches the use of using finely divided carbon particles such as carbon black as the substrate for small (tens of angstroms) particles of the noble metal. Thus called a xe2x80x9csupportedxe2x80x9d catalyst, this methodology has shown superior performance and utilization of: the catalyst in electrochemical applications. However, while supported platinum catalysts have demonstrated high activity for hydrogen oxidation, this proclivity for facile kinetics is severely retarded with carbon monoxide concentrations of only a few ppm.
Thus, with a fuel processor technology producing hydrogen streams containing around 50 ppm CO and platinum-based gas diffusion anodes being poisoned slowly with as little as 1 ppm, there is a clear need for a CO tolerant catalyst. The current state-of-the-art CO tolerant electrocatalyst is a platinum ruthenium bimetallic alloy (Pt:Ru) and is available commercially in supported form (E-TEK, Inc., Natick, Mass.). The mechanism for CO tolerance is believed to involve the nucleation of oxygen containing species (OHads) on the ruthenium site such that platinum-adsorbed CO can participate in a bimolecular reaction with the activated oxygen thereby freeing the platinum site for hydrogen oxidation. However, the ruthenium site is also prone to poisoning by CO at higher concentrations of CO, and the important nucleation of oxygen containing species is then inhibited (H. A. Gasteiger, N. M. Markovic, and P. N. Ross; J. Physical Chemistry, Vol. 99, No. 22, 1995, p 8945). Although Pt:Ru has been optimized and thoroughly studied to show that an alloy composed of Pt:Ru in the atomic ratio of 1:1 yields the best tolerance to CO, this bimetallic catalyst functions only at around 10 ppm CO or less because of the eventual poisoning of the ruthenium site.
A recent monograph reviewing bimetallic electrocatalysts has summarized several important facts in the preparation and activity of electrocatalysts (P. N. Ross: xe2x80x9cThe Science of Electrocatalysis on Bimetallic Surfaces,xe2x80x9d in Frontiers in Electrochemistry Vol. 4, J. Lipowski and P. N. Ross Jr., Wiley-Interscience, New York, N.Y., 1997). The activity of a bimetallic catalyst is dependent on electronic and structural effects. Electronic properties are determined by the electron configuration of the alloying elements while structural properties are determined by both the selection of alloying elements and the method of preparation of the alloy itself. This last observation is important in the design of CO tolerant catalysts. For example, a Pt:Ru alloy prepared by sputtering a bulk alloy, annealing a bulk alloy, or depositing a submonolayer of ruthenium on platinum all yield fundamentally different catalytic properties (P. N. Ross, p 19). The precept that alloy formation methodology influences catalyst function follows from the creation of three zones in every bimetallic catalyst: metal xe2x80x9cAxe2x80x9d, metal xe2x80x9cBxe2x80x9d, and an intermixed zone xe2x80x9cA-Bxe2x80x9d. The distribution of these zones determines activity.
Another important property noted by Ross in the monograph is that the phenomenon of surface segregation in bimetallic alloys has often been neglected. Surface segregation is the enrichment of one element at the surface relative to the bulk, and in our case would be dominated by platinum in an alloy of 4d elements with the exception of silver and tin (Ross, p. 51).
In summary, there is ample evidence to show that electrocatalysts can differ in their activity due to preparation methods. Another difference arises from dissimilarities between the bulk and surface compositions of the alloy. For these two reasons, we expect even greater contrasts to occur between bimetallic alloys prepared as bulk metals compared to alloys prepared as very small (10 to 300 xc3x85) supported particles.
Molybdenum has been observed to play a catalytic role in the oxidation of small organic molecules otherwise known as xe2x80x9cC1xe2x80x9d molecules (to designate one carbon atom). As early as 1965, a molybdenum platinum black complex was implicated in the catalytic oxidation of formaldehyde and methanol in sulfuric acid (J. A. Shropshire; Journal of the Electrochemical Society, vol. 112, 1965, p. 465). Although the molybdenum was added as a soluble salt, it was reduced and deposited onto the platinum black electrode. Later on, several others took note of this property of molybdenum and tried to intentionally create platinum alloys. H. Kita et al. confirm that a platinum molybdenum complex formed through reduction of the metal salt onto the surface of the platinum foil electrode can catalyze methanol oxidation (H. Kita et al.; J. Electroanalytical Chemistry, vol. 248, 1988, p. 181). H. Kita extended this work to creating a membrane electrode assembly (MEA) of chemically deposited platinum and molybdenum on Nafion, to be used in a PEMFC. As before, the fuel here is methanol (H. Kita et al.; Electrochemistry in Transition, Oliver Murphy et al., Eds., Plenum Press, New York, 1993, p. 619). These are both examples of forming an alloy through deposition of a submonolayer of molybdenum onto platinum although no high surface area support is used.
Masahiro Watanabe discloses the use of vacuum sputtering to form an alloy of Ni, Co, Mn or Au with Pt, Pd, or Ru. The object of this patent is to provide a CO tolerant anode catalyst for the PEMFC (Masahiro Watanabe: Japan Patent Application No. H6-225840, Aug. 27, 1994). Although this patent directs towards a preferred alloy consisting of Pt with Ni, Co, Mn, or Au, a comparison example of Pt with Mo is shown whereby sustained currents for hydrogen oxidation in the presence of CO dissolved in sulfuric acid are recorded. The example employs a rotating disk electrode coated with an alloy formed by simultaneous argon sputtering under reduced pressure. While the patent emphasizes the use of sputter coating, some mention is made to carbon supported alloys prepared by the usual thermal decomposition methods. However, there is no description or teaching as to how the properties achieved in a sputter-coated alloy could be obtained by thermal decomposition onto carbon black.
A recent publication indicates the potential for Pt:Mo as a CO tolerant catalyst superior to Pt:Ru (B. N. Grgur et al.; Journal of Physical Chemistry (B), vol. 101, no. 20, 1997, p. 3910). In this paper, a sample of Pt75Mo25 alloy is prepared as a bulk crystal by arc melting of the pure elements in an argon atmosphere and homogenizing with a heat treatment. The authors show that the resulting boule possessed a uniform metal alloy composition from the interior bulk to the surface. This well characterized surface is formed into a rotating electrode disk and shows oxidation of hydrogen in a mixed gas of H2/CO. The authors put forth evidence that the molybdenum may participate in a greater rate of CO oxidation compared to the ruthenium. Furthermore, the authors point out that ruthenium and platinum do not differ much in that they both absorb H2 and CO, possess quasireversible OHads states, and are electrocatalysts for H2 and CO: the alloying process does not produce a fundamental change in the properties of either metal. On the other hand, molybdenum is significantly different than platinum and formation of the alloy produces a material with substantial differences in the intrinsic chemical properties. While the authors relate a surface with unexpected catalytic properties, there is no mention of how one could translate the properties discovered in this bulk alloy to the highly disperse carbon supported catalysts employed in gas diffusion electrodes.
There has been some effort in the patent literature to create the supported Pt:Mo alloy on carbon blacks. Landsman et al. in U.S. Pat. No. 4,316,944 describe a method to form noble metal chromium alloys on carbon black for eventual incorporation into a cathode of a fuel cell. In this case, the inventors were seeking superior oxygen reduction catalysts for use with PAFC. They make use of a powder of already-dispersed platinum on metal and a solution of ammonium chromate. The addition of dilute hydrochloric acid was added to cause the adsorption of the chromium species on the supported catalyst. Heat treatment in nitrogen was used to form the platinum chromium alloy. Although Pt:Mo appears in a table of results as a cathode catalyst, no details are given to its preparation, metal:metal ratio, or metal on carbon weight loading.
Thus, there is a need to show a method of preparation and formulation requirements that preserve the unexpected CO tolerant properties of Pt:Mo on carbon black supports that would then allow this alloy to be readily incorporated into gas diffusion electrodes or membrane electrode assemblies (MEAs).
It is an object of this invention to provide an improved high-surface area formulation of platinum:molybdenum on a carbon support whereby: the bulk atomic ratio of Pt:Mo is between 99:1 and 1:1, preferably between 3:1 and 5:1, and more preferably 4:1; and the metal loading of alloy on carbon support is between 1% and 80% total metal on carbon, preferably between 20% and 40%.
It is a further object of this invention to provide an anode catalyst for a fuel cell whereby hydrogen can be oxidized in the presence of carbon monoxide.
It is also an object of this invention to provide a method of manufacturing supported platinum molybdenum alloy with highly desirable surface activity.
It is a final object of this invention to provide an anode catalyst with high activity for the direct oxidation of small organic molecules such as methanol.
Amongst the aforementioned methods of forming a bimetallic alloy, we have found that a combination of deposition and bulk annealing forms the most potent form of the alloy. As has been previously established, the precipitation of metal salts onto carbon black supports can yield highly disperse formulations of metal. For example, through the teachings of Petrow and Allen, a complex of platinum sulfite acid produces extremely small and well-dispersed particles of platinum on carbon black. The Table below illustrates the relationship between weight loading on carbon black (here Vulcan XC-72), the resulting average platinum crystallite size, and the effective platinum surface area.
Reproduced from E-TEK, Inc. Gas Diffusion Electrodes and Catalyst Materials, Catalog, 1998, p 15.
While there are clear trends with regards to particle size and effective surface area, it is important to note that the specific activity of the catalyst follows a trend as well. As reviewed by Markovic, Gasteiger, and Ross in The Journal of the Electrochemical Society, Vol. 144, No. 5, May 1997, p 1591, the oxygen reduction rate and hence activity of platinum can be highly sensitive to the type and abundance of crystal face (111, 100, and 110). Furthermore, Markovic et al. point that the platinum crystallite size controls the relative abundance of the various face geometries. Since the activity of a CO tolerant alloy depends on the final structure of the alloy crystal, control of metal loading, particle size, and distribution of particle size all play a vital role as well as the actual method of alloy formation.
In one preferred embodiment, manufacture of platinum-molybdenum alloys begins by first depositing platinum on a carbon black. Colloidal particles of Pt oxide are deposited on a carbon support from an aqueous solution of a platinum precursor containing the support material. In order to form a colloid, the platinum containing species can be subjected to an oxidizing agent or the solution can be simply evaporated. Although Pt sulfite acid is the preferred choice for the precursor, chloroplatinic acid could alternatively be used. In a second step, discrete particles of Mo oxide are deposited on the Pt oxide containing carbon support by adsorption of colloidal Mo oxide or Mo blue, formed in situ by mild reduction of a solution containing a Mo precursor, for instance an ammonium molybdate solution or a solution containing Mo with alkali hydroxide. Several chemical reducing agents may be employed as well known to one skilled in the art, for example hydrazine, formic acid, formaldehyde, oxalic acid, hydrogen or metals having a sufficiently low potential such as molybdenum and zinc: another method for reducing the Mo containing solution consists in feeding said solution to an electrochemical cell, applying direct current thereto and reducing the Mo precursor at the cathode. After drying, the catalyst is first subjected to a reducing atmosphere between 500 and 900xc2x0 C., and then alloyed at higher temperature (for instance at 900 to 1200xc2x0 C.) in the same reducing atmosphere or in an inert one: in one preferred embodiment, it may be reduced at 500-800xc2x0 C. in H2 gas, then heat treated at 800-1200xc2x0 C. in Ar gas to form the alloy phase of Pt and Mo. In another preferred embodiment, reduction and alloying are both performed in a H2 environment between 500 and 1200xc2x0 C., either in a single or in two subsequent temperature steps. This general method is applicable to preparations of Pt:Mo alloys supported on amorphous and/or graphitic carbon materials with a ratio of Mo alloyed with Pt from 1 to 50 atomic % and a total metal loading on the carbon support from 1-90%. It is however preferred that the total metal loading be comprised between 10 and 40%. This method produces a carbon supported Pt:Mo alloy catalyst with a metal particle size of approximately 300 xc3x85 or less.
Other methods for preparing carbon supported Pt:Mo alloys of the same characteristics will be given in detail in the following examples.
Catalysts produced in this manner are readily incorporated into gas diffusion electrodes For example Pt:Mo catalysts thus prepared can be incorporated into structures similar to the commercially available ELAT(copyright) (E-TEK, Inc., Natick, Mass.). Here, a carbon cloth serves as the web. A layer of Shawinigan Acetylene Black (SAB) mixed with polytetrafluoroethylene binder (e.g. Teflon(copyright) commercialized by DuPont, Wilmington, Del.) serves as the wetproofing layer on each side of the web. Finally, layers of carbon black such as Vulcan XC-72 with the alloy Pt:Mo are coated onto one side of the assembly: preferably, the specific loading of metal with respect to the active area is comprised between 0.1 and 5 mg/cm2. After the final coat, the assembly may be sintered in air at a temperature sufficient to cause the binder to flow, typically 300-350xc2x0 C. Allen et al. in U.S. Pat. No. 4,293,396 further describe the construction of this type of gas diffusion electrode. Such catalysts can also be incorporated in other gas diffusion electrode structures, for example the electrodes in co-pending patent xe2x80x9cImproved Structures and Methods of Manufacture for Gas Diffusion Electrodes and Electrode Componentsxe2x80x9d are suitable as well as described in U.S. provisional application Serial No. 60/070,342 filed Jan. 2, 1998.
These carbon-supported alloys can also be deposited onto the surface of an ion conducting membrane such as Nafion(copyright) or Gore Select(copyright) commercialized respectively by DuPont and Gore and Associates, Elkton, Md. Wilson and references therein have described methods for such operations in U.S. Pat. No. 5,234,777. In general, depositing the catalyst on the membrane through a xe2x80x9cdecalxe2x80x9d method (seexe2x80x94Wilson) can create a membrane electrode assembly, or one can apply a paint or ink of catalyst to the membrane, or a catalyzed gas diffusion electrode can be mechanically or heat-pressed against the membrane.
For the examples listed here, we have employed a catalyzed gas diffusion electrode similar to that described in Allen et al. pressed against a Nafion membrane. However, fuel cell tests can be highly dependent on system configuration. For example, the mechanical geometry one uses to make contact between the electrode and the membrane, the flow field geometry employed to feed gasses to anode and cathode, and the method and manner of providing hydrated gasses to the cell can all affect the cell performance. In order to evaluate catalyst performance in the absence of system variables but still as an active component of a gas diffusion electrode, we also employ a simple three-electrode test method.
The three-electrode or xe2x80x9chalf cellxe2x80x9d method fits 1 cm2 sample of gas diffusion electrode into an inert holder. The gas-feed side of the gas diffusion electrode is positioned into a plenum whereby an excess of oxygen, air, hydrogen, or hydrogen containing levels of CO is passed at low pressures (on the order of 10 mm of water or less). The face containing the catalyst (that would normally be against the membrane of a PEMFC) is held in a 0.5M H2SO4 solution at a fixed temperature. The counter electrode is placed directly across the working electrode, and a reference electrode is held in-between the two. The fixed geometry is maintained between the three electrodes through a specially constructed cap. A potentiostat is employed to control the potential and measure the current.